I always wonder the same thing. I guess it's like 'severe icing': not supposed to fly through it, but you'll never get re-routed or cancelled for it. I asked a dispatcher once (when he came into our ground school) how he deals with severe icing- he had no answer, almost like he never had even heard of it.

Turbulence should be taken more seriously by many, and technically severe turbulence requires a write up and maintenance inspection. I also think that turbulence gets over-estimated sometimes- at least going by the text-book definitions.

Fly slow and speak up if the guy next to you thinks 320 in turbulence is just fine. Of course I don't understand why anyone flies that fast at all. We get paid by the hour.